1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * Verification of builtin signatures
  4 *
  5 * Copyright 2019 Google LLC
  6 */
  7
  8/*
  9 * This file implements verification of fs-verity builtin signatures.  Please
 10 * take great care before using this feature.  It is not the only way to do
 11 * signatures with fs-verity, and the alternatives (such as userspace signature
 12 * verification, and IMA appraisal) can be much better.  For details about the
 13 * limitations of this feature, see Documentation/filesystems/fsverity.rst.
 14 */
 15
 16#include "fsverity_private.h"
 17
 18#include <linux/cred.h>
 19#include <linux/key.h>
 20#include <linux/security.h>
 21#include <linux/slab.h>
 22#include <linux/verification.h>
 23
 24/*
 25 * /proc/sys/fs/verity/require_signatures
 26 * If 1, all verity files must have a valid builtin signature.
 27 */
 28int fsverity_require_signatures;
 29
 30/*
 31 * Keyring that contains the trusted X.509 certificates.
 32 *
 33 * Only root (kuid=0) can modify this.  Also, root may use
 34 * keyctl_restrict_keyring() to prevent any more additions.
 35 */
 36static struct key *fsverity_keyring;
 37
 38/**
 39 * fsverity_verify_signature() - check a verity file's signature
 40 * @vi: the file's fsverity_info
 41 * @signature: the file's built-in signature
 42 * @sig_size: size of signature in bytes, or 0 if no signature
 43 *
 44 * If the file includes a signature of its fs-verity file digest, verify it
 45 * against the certificates in the fs-verity keyring. Note that signatures
 46 * are verified regardless of the state of the 'fsverity_require_signatures'
 47 * variable and the LSM subsystem relies on this behavior to help enforce
 48 * file integrity policies. Please discuss changes with the LSM list
 49 * (thank you!).
 50 *
 51 * Return: 0 on success (signature valid or not required); -errno on failure
 52 */
 53int fsverity_verify_signature(const struct fsverity_info *vi,
 54			      const u8 *signature, size_t sig_size)
 55{
 56	const struct inode *inode = vi->inode;
 57	const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
 58	struct fsverity_formatted_digest *d;
 59	int err;
 60
 61	if (sig_size == 0) {
 62		if (fsverity_require_signatures) {
 63			fsverity_err(inode,
 64				     "require_signatures=1, rejecting unsigned file!");
 65			return -EPERM;
 66		}
 67		return 0;
 68	}
 69
 70	if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
 71		/*
 72		 * The ".fs-verity" keyring is empty, due to builtin signatures
 73		 * being supported by the kernel but not actually being used.
 74		 * In this case, verify_pkcs7_signature() would always return an
 75		 * error, usually ENOKEY.  It could also be EBADMSG if the
 76		 * PKCS#7 is malformed, but that isn't very important to
 77		 * distinguish.  So, just skip to ENOKEY to avoid the attack
 78		 * surface of the PKCS#7 parser, which would otherwise be
 79		 * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
 80		 */
 81		fsverity_err(inode,
 82			     "fs-verity keyring is empty, rejecting signed file!");
 83		return -ENOKEY;
 84	}
 85
 86	d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
 87	if (!d)
 88		return -ENOMEM;
 89	memcpy(d->magic, "FSVerity", 8);
 90	d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
 91	d->digest_size = cpu_to_le16(hash_alg->digest_size);
 92	memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
 93
 94	err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
 95				     signature, sig_size, fsverity_keyring,
 96				     VERIFYING_UNSPECIFIED_SIGNATURE,
 97				     NULL, NULL);
 98	kfree(d);
 99
100	if (err) {
101		if (err == -ENOKEY)
102			fsverity_err(inode,
103				     "File's signing cert isn't in the fs-verity keyring");
104		else if (err == -EKEYREJECTED)
105			fsverity_err(inode, "Incorrect file signature");
106		else if (err == -EBADMSG)
107			fsverity_err(inode, "Malformed file signature");
108		else
109			fsverity_err(inode, "Error %d verifying file signature",
110				     err);
111		return err;
112	}
113
114	err = security_inode_setintegrity(inode,
115					  LSM_INT_FSVERITY_BUILTINSIG_VALID,
116					  signature,
117					  sig_size);
118
119	if (err) {
120		fsverity_err(inode, "Error %d exposing file signature to LSMs",
121			     err);
122		return err;
123	}
124
125	return 0;
126}
127
128void __init fsverity_init_signature(void)
129{
130	fsverity_keyring =
131		keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
132			      current_cred(), KEY_POS_SEARCH |
133				KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
134				KEY_USR_SEARCH | KEY_USR_SETATTR,
135			      KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
136	if (IS_ERR(fsverity_keyring))
137		panic("failed to allocate \".fs-verity\" keyring");
138}